On Tue, 3 Dec 2019 at 15:30, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > > On Mon, Dec 2, 2019 at 4:41 PM Matthew Garrett > <matthewgarrett@xxxxxxxxxx> wrote: > > > > Add an option to disable the busmaster bit in the control register on > > all PCI bridges before calling ExitBootServices() and passing control to > > the runtime kernel. System firmware may configure the IOMMU to prevent > > malicious PCI devices from being able to attack the OS via DMA. However, > > since firmware can't guarantee that the OS is IOMMU-aware, it will tear > > down IOMMU configuration when ExitBootServices() is called. This leaves > > a window between where a hostile device could still cause damage before > > Linux configures the IOMMU again. > > > > If CONFIG_EFI_NO_BUSMASTER is enabled or the "disable_busmaster=1" > > commandline argument is passed, the EFI stub will clear the busmaster > > bit on all PCI bridges before ExitBootServices() is called. This will > > prevent any malicious PCI devices from being able to perform DMA until > > the kernel reenables busmastering after configuring the IOMMU. > > I hate to be an obnoxious bikeshedder, but I really dislike the > "disable_busmaster" name. I read this and $SUBJECT as "for some > reason, the admin wants to operate the system with busmastering off". > What you really want is something more like "disable busmastering > before IOMMU initialization". Maybe > "iommu.disable_busmaster_before_init"? > > Similarly, EFI_NO_BUSMASTER sounds like a permanent state of affairs. > > Would a similar patch apply to non-EFI boot? That is, in a BIOS boot, > is busmastering on when the kernel is loaded? > Yes, bus mastering is on, but since legacy BIOS may implement things like PS/2 emulation or other compatibility hacks where the PCI masters (devices or bridges) may need to be left enabled across the transition from firmware into the OS, I don't think it is wise to try and implement this feature for it. So the EFI stub is a reasonable place to put a feature like this, except for the fact that [on x86], it does not get invoked unless GRUB boots your kernel with 'linuxefi' rather than 'linux', and so in the majority of cases, I guess we are essentially doing legacy BIOS boot, even on UEFI systems.
